U.S. Pat. No. 5,103,367, entitled Electrostatic Chuck Using A.C. Field Excitation relates to a mechanism for holding semiconductor wafers in contact with a support during treatment of the wafers. The electrostatic chuck has three electrodes where two electrodes define a substantially planar surface and are embedded within a thin dielectric film. These two electrodes are excited by a low-frequency (approximately 200 hertz) A.C. supply to produce sign wave fields of controlled amplitude and phase. The third electrode accesses a shield electrode that serves as a reference point for the other two electrodes. By controlled rates of voltage application and removal, low-voltage gradients are obtained on the wafer support. This results in no retentive forces between the dielectric medium and the wafer. A low alternating current amplitude excitation of the chuck enables capacitative current sensing of the relative positions of the wafers and the dielectric film enabling simple control of voltage application to the electrodes.
A common problem associated with electrostatic clamping is that the wafer may not release from the clamp when the holding voltage that is applied to the clamp is turned off. There are several factors which can contribute to this residual clamping force.
First, most dielectrics can exhibit a degree of semipermanent polarization following exposure to an intense electric field. This polarization has the effect of maintaining the clamping force even after the applied voltage is removed. Materials with a high dielectric constant are particularly prone to this effect, and it is exactly such materials which are favored as the dielectric material for a wafer support because their high dielectric constant provides an enhancement of the clamping force.
Second, stored elastic energy in the clamp can result in a residual clamping force, even were the dielectric a perfect material without any residual polarization. For example, in the case where an elastomer is part of the clamp structure, the capacitance of the clamp becomes voltage dependent. This makes discharge of the series of capacitors which comprise the clamp impossible by simple shorting the voltage supply. Rather, the stored energy must be dissipated in an external circuit. Because the clamp circuitry is not a simple D.C. source but contains capacitance and inductance, simple shorting through an external circuit to extract the stored energy will not work.